This invention relates to a hollow blow-molded bottle-shaped container of biaxially oriented polyethylene terephthalate resin.
Polyethylene terephthalate resin, hereinafter referred to as "PET", has a variety of excellent properties and characteristics. In order to take advantage of the excellent properties and characteristics of PET, a bottle-shaped container made of PET is usually biaxially oriented.
Although the conventional hollow blow-molded bottle-shaped container of biaxially oriented PET has a variety of advantageous properties and characteristics as described above, the container does not always exhibit a sufficiently strong gas barrier property.
In order to supplement the insufficient gas barrier property of the bottle-shaped container of PET, it has been considered heretofore to provide a hollow blow-molded bottle-shaped container of biaxially oriented PET with a multilayer structure including, in combination, a layer formed of a synthetic resin material having excellent gas barrier properties and a layer formed of PET. However, the adhering strength between the layer formed of the material having excellent gas barrier properties and the layer formed of PET is weak or insufficient because of differences in cooling shrinkage factors, at the molding time, between the layer formed of the material having excellent gas barrier properties and the layer formed of PET and of insufficient compatibility, bonding strength and reaction strength between the material having excellent gas barrier properties and PET. Thus, the mechanical strength of such bottle-shaped containers is insufficient and gas, such as carbon dioxide, may escape from the contents of the container and accumulate between the layer formed of the material having excellent gas barrier properties and the intermediate layer formed of PET.
In other words, when a performed injection molded piece is covered with a surface layer in which both inner and outer surfaces of an intermediate layer formed of a material having high gas barrier properties are covered with PET, the surface layer and the intermediae layer are bonded relatively strongly in the preformed injection molded state. However, when the preformed piece is further blow-molded to biaxially orient the preformed piece into a bottle-shaped container, the bonding strength between the surface layer and the intermediate layer becomes extremely weak due to the differing degrees of orientation beweeen the surface layer and the intermediate layer of the bottle-shaped container.
Accordingly, the carbon dioxide gas that penetrates into the inner surface layer is readily accumulated between the inner surface layer and the intermediate layer as described above. In this manner, an exfoliation between the inner surface layer and the intermediate layer occurs over the entire bottle-shaped container, and the body of the container tends to swell, since the body of the container cannot withstand the internal pressure produced in the container by the outer surface layer and the intermediate layer due to the internal pressure of the container, resulting in a substantial deterioration of the external appearance of the bottle-shaped container as a merchandise item.
A further drawback of the resin material which has excellent gas barrier properties is that the material whitens when it contacts water. When a gap is formed between the inner surface layer and the intermediate layer of the bottle-shaped container as described above, the intermediate layer is whitened due to water which penetrates to a very small degree into the gap formed between the inner surface layer and the intermediate layer, thereby resulting in a decrease of transparency of the bottle-shaped container.